1. Field of the Invention
The invention relates to a multistep process for preparing organopolysiloxanes by the hydrolysis and condensation of chlorosilanes, and also to an apparatus in which the first process step can advantageously be carried out.
2. Description of the Related Art
Processes for preparing organopolysiloxanes are already known. As prior art closest to the present invention, reference may be made to DE 102007004838 A1 and references cited therein.
DE 102007004838 A1 discloses how the molecular weight, and thus further product properties of organopolysiloxanes prepared by hydrolysis can be controlled while maintaining other process parameters by means of the of pressure and temperature selected in the first process step. The other process parameters, in particular, the stoichiometry of the starting materials, reaction times, etc., are kept constant since, according to the prior art, they likewise influence the properties of the product.
The first process step is carried out in a simple, continuously operated reaction loop (see FIG. 1) which is provided with feed lines for the reactants, a degassing vessel for separating off the HCl and also an overflow. The reaction product which is obtained continuously (“partial alkoxylate”) is temporarily stored in a buffer vessel before it is used in the subsequent, discontinuous hydrolysis step.
A disadvantage of the process thus described is that a large part of the hydrogen chloride liberated in the hydrolysis is not obtained in gaseous form and therefore cannot be passed to HCl recycling (reaction with methanol to form methyl chloride). The intermediate (“partial alkoxylate”) obtained in the first step still contains a considerable amount of alcohol which has not reacted with chlorosilane and also hydrogen chloride (“HCl”) dissolved therein. This is undesirable since the HCl gas dissolved in the alcohol has to be diluted further with water in the subsequent 2nd step of the reaction sequence and can no longer be recirculated economically to the HCl circuit. In addition, it has to be neutralized with sodium hydroxide before disposal, which is associated with further costs. The NaCl formed is subsequently introduced into public bodies of water and pollutes the environment.